The present invention relates to a multiple filter unit for purifying air in closed environments. The filter unit is insertable into ventilation and air conditioning systems. It can also advantageously be used in motor vehicles.
The industrialization of the world has resulted in extensive atmospheric pollution. Any type of combustion, e.g. from industrial plants, motorization or general heating, leads to final products which have to be dispersed or settled. These are constituted by, among all, oxygenated carbon compounds, unburned hydrocarbons, nitrogen oxides, oxygenated sulphur compounds, water vapor and unbuned or incombustible solid particles. It can be a health hazard to breathe in certain areas where the traffic is dense and/or the industrial pollution is extensive. This is also often the case in motor vehicle coupes, especially in traffic jams.
Inside modern buildings, other noxious polluting substances which are released from the furniture pieces, wall coatings and the like, may be present.
Hence, there is a great need of methods for removing different sorts of polluting substances from air.
Various pollutant removal methods are available in the state of the art, such as:
a) mechanical filtering (removal of solid particles); PA1 b) conversion of gaseous pollutants into other products and recovery of the new substances; PA1 c) decomposition of gaseous pollutants and conversion into harmless products; PA1 d) conversion into easily eliminated products; PA1 e) elimination of gaseous pollutants by cooling and condensation; and PA1 f) elimination by absorption/adsorption. PA1 a) an inert separation filter for removing solid particulate matter, such as dust particles; PA1 b) a filter element comprising solids that adsorb large molecules by using van der Walls forces such as activated carbon; PA1 c) a filter element having sufficiently small pores (molecular sieves) to prevent tiny uncharged molecules such as short hydrocarbons from penetrating the element, such as an element made of zeolites; and PA1 d) a filter element having the ability of binding molecules containing polar functional groups.
A lot of pollutant-removal methods rely upon different kinds of absorption and adsorption. EP-0 162 022 relates to an interchangeable multiple filter unit, especially for motor vehicles and air conditioning. The filter unit comprises mechanical and absorbing filter elements. Those absorbing elements comprises gas absorbing materials such as activated carbon. The filter unit also comprise a catalyst for converting CO to CO.sub.2.
Filter systems based upon adsorption by activated carbon are widely used for removing polluting gases and vapors from air. The vander Waals' forces of the carbon particles lead to adsorption of the polluting substances. However, such filter systems do not remove gaseous pollutants to a satisfying extent. Especially small uncharged molecules, such as hydrocarbons, and a major fraction of molecules having certain functional groups such as NOX, SOX, aldehydes, etc. will pass through the filter unit and contaminate the purified air. Moreover, if molecules adsorbed on active carbon are exposed to oxidizing compounds such as ozone, these compounds can be converted to other compounds not binding to active carbon. An example of such compounds is unsaturated hydrocarbons which are converted into releasable aldehydes by ozone.
EP-A2-0 398 847 relates to an air filter comprising a mechanical filter for removing solid particles, an adsorption filtering layer consisting of activated carbon and two chemical filtering layers flanking the mechanical filter on both sides. One of these layers is acidic and the other one is basic. This filter can absorb a larger range of contaminating substances than the filter according to EP-0 162 022, but small uncharged molecules without acidic or basic groups will also pass through this filter.
Polyimide has been shown to possess interesting properties in non-cryogenic gas separation processes. EP-0 385 240 describes a polyimide membrane that can be used in the process of recovering O.sub.2 from an O.sub.2 /N.sub.2 stream or air. U.S. Pat. No. 4 892 719 discloses a glass fiber furnace filter coated with a polymeric amine such as polyethylene imine. According to this document, indoor pollutants such as formaldehyde, or acidic gases such as SO.sub.2, NO.sub.2, and H.sub.2 S can be removed from a house by a reactive method consisting of a coating on a furnace filter in a forced-air heating system.
However, these polyimine filters have the drawback of not adsorbing polluting molecules lacking polar functional groups. Thus, only a very narrow range of molecules can be adsorbed by such a filter.
EP-A1-0 583 594 discloses a system and a method for removing hydrocarbons from a gaseous mixture where the system comprises different zeolites acting as molecular sieves. Molecules having a larger size than a certain predetermined value are trapped inside the zeolite particle. The system according to EP-0 583 594 is very efficient and consequently, it is easy to fill the zeolite particles with contaminating gas molecules as well as solid particles. If the filter shall be used for a long time, it is essential that the air that is to be filtered already is purified to a large extent. Zeolites are expensive and to use a filter system according to EP-0 583 594, where it often is necessary to change the zeolites in order to maintain sufficient adsorption can lead to very high costs.
Currently, no filter construction has the ability of satisfying purifying polluted air comprising a large amount of different polluting substances, such as air containing cigarette smoke. The same applies to air containing smelling substances.
Consequently, there is a need for an air filter having the ability to absorb a broad range of contaminating substances from solid particles to small hydrocarbons without causing unnecessarily high operation costs.